Electrochemical cells are presently the preferred method of providing cost effective portable power for a wide variety of consumer devices. The consumer device market dictates that only a handful of standardized cell sizes (e.g., AA, AAA or AAAA) and specific nominal voltages (e.g., 1.5 V) be provided, while more and more consumer electronic devices, such as digital still cameras, are being designed with relatively high power operating requirements. Despite these constraints, consumers often prefer and opt to use primary batteries for their convenience, reliability, sustained shelf life and more economical per unit price as compared to currently available rechargeable (i.e., secondary) batteries.
Within this context, it is readily apparent that design choices for primary non-rechargeable) battery manufacturers are extremely limited. For example, specified, nominal voltages for standard consumer products significantly limits the selection of potential electrochemical materials in batteries, while the use of standard cell sizes restricts the overall available internal volume available for active materials, safety devices and other elements. Taken together, these choices have made 1.5 V primary battery systems, such as alkaline or lithium-iron disulfide systems, far more prominent than others, such as 3.0 V and higher lithium-manganese dioxide.
However, considerations in designing 1.5V primary batteries are significantly different are considerably different than higher voltage and/or rechargeable systems. For example, some battery chemistries (e.g., alkaline and nickel oxy-hydroxide) rely on an aqueous and highly caustic electrolyte that has a propensity for gas expansion and/or leakage, leading to designs, in terms of selection of internal materials and/or compatibility with containers and closures, that are very different than other chemistries (e.g., lithium-iron disulfide). Further, in rechargeable 1.5 V systems (note that lithium-iron disulfide systems are not currently considered suitable for consumer-based rechargeable systems), highly specialized electrochemical compositions are used, although these high cost components are deemed acceptable because secondary systems typically sell for a higher retail price than their primary battery equivalents. Additionally, the discharge mechanisms, cell designs and safety considerations of rechargeable systems are, by and large, inconsequential and/or inapplicable to primary systems.
Within the realm of 1.5 V systems, lithium-iron disulfide batteries (also referred to as LiFeS2, lithium pyrite or lithium iron pyrite) offer higher energy density, especially at high drain rates, as compared to alkaline, carbon zinc or other 1.5 V battery chemistries. But even with the inherent advantages of lithium-iron disulfide cells for high power devices, LiFeS2 cell designs must strike a balance between the cost of materials used, the incorporation of necessary safety devices and the overall reliability, delivered capacity and expected conditions for use of the cell (e.g., temperature, drain rate, etc.). Normally, low power designs emphasize the quantity of active materials, while high power designs focus more on configurations to enhance discharge efficiency. As an added consideration, iron-disulfide's propensity to expand significantly during discharge, in comparison to other batteries, coupled with its relatively hard and granular nature in slurry- and/or roll-coated electrodes, puts extreme stresses on the separator and other cell components during discharge which, if the compromised, could cause the cell to fail to deliver its designed capacity.
Safety devices, such as venting mechanisms and thermally activated “shutdown” elements, and improvements in discharge reliability (e.g., by preventing internal short circuits) occupy internal cell volume and involve design considerations that are usually counterproductive to cell internal resistance, efficiency and discharge capacity. An additional challenge is presented by transportation regulations, which limit the amount of weight lithium batteries can lose during thermal cycling—meaning that cell designs for smaller container sizes like AA and AAA can only lose milligrams of total cell weight (usually by way of evaporation of the electrolyte). Lastly, the reactive and volatile nature of the non-aqueous, organic electrolytes required for LiFeS2 cells severely limits the universe of potential materials available (particularly with respect to interactions between the electrolyte and cell closure, separator and/or current collector(s) provided within the cell) as compared to other electrochemical systems.
With these considerations in mind, a lithium-iron disulfide battery is desired which will reliably deliver its fully designed discharge capacity at temperatures exceeding 70° C. is desired.